Using NASA's Chandra X-ray Observatory, astronomers have identified a hidden companion, either a stellar-mass black hole or neutron star, that is pulling material from a massive star every 113 days, triggering powerful outbursts that mimic supernovae. The discovery, published in The Astrophysical Journal, provides the first X-ray detection of such an erupting star system and offers an explanation for the 15-year puzzle surrounding the volatile star AT 2016blu, located 30 million light-years away in the spiral galaxy NGC 4559.
The star AT 2016blu is a luminous blue variable, a type of hot, massive star that undergoes dramatic brightness changes. Because its eruptions resemble supernova explosions but do not destroy the star, it is classified as a supernova impostor. Between 2012 and 2026, astronomers recorded at least 27 optical outbursts from the star, occurring roughly every 113 days.
The cause remained unclear until researchers obtained Chandra X-ray data timed with an outburst in March 2026. The observations showed the system was at least 100 times brighter in X-rays than before the outbursts began, indicating material from the star is being pulled onto a compact companion, either a black hole or neutron star, during each close orbital pass. This is the first time a black hole or neutron star companion has been found for a star of this type.
“For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis,” said Mojgan Aghakhanloo of the University of Virginia, who led the study. “Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen.” Co-author Núria Torres-Albà, also of the University of Virginia, said: “This star’s behavior was confusing at first because it kept acting like a supernova, but clearly it’s not because it keeps surviving these outbursts about three times a year.” Nathan Smith of the University of Arizona, another co-author, said: “We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays.”
The X-ray brightness qualifies AT 2016blu as a candidate ultra-luminous X-ray source, a class of objects that are not well understood. The finding suggests a link between these sources and violent stellar eruptions, and may help studies of stars that change in brightness before they undergo supernova explosions.
Analysis
Why This Matters
- Explains the mechanism behind a mysterious class of eruptive stars, solving a puzzle that has endured for over a decade.
- Provides the first X-ray detection linking a supernova impostor to a black hole or neutron star companion, opening a new window into studying these extreme binary systems.
- May help uncover the nature of ultra-luminous X-ray sources, which are among the brightest X-ray objects in the universe.
Background
Luminous blue variables are massive, hot stars known for irregular brightness changes. Some, like Eta Carinae, undergo extreme eruptions. AT 2016blu is a supernova impostor: its outbursts are as bright as supernovae but do not destroy the star. Until now, the driving force behind these repeated eruptions was unknown. The new Chandra observations reveal that a compact companion (a black hole or neutron star) is responsible, pulling material from the giant star during their 113-day orbital cycle.
What to Watch
- Future Chandra observations may determine whether the companion is a black hole or a neutron star, based on detailed X-ray variability.
- The system could become a test bed for models of mass transfer in massive star binaries and their evolution toward supernova.
- Searches for similar X-ray counterparts in other supernova impostors could reveal how common such systems are.
Checked
12 claims in this story checked against 1 source: 11 supported.
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- SupportedChandra uncovered a hidden companion that is causing the star’s periodic eruptions.NASA: “Using NASA’s Chandra X-ray Observatory, astronomers have discovered what is triggering repeated outbursts from a giant, unstable star. A new study, published Monday in the latest issue of The Astrophysical Journal, details the discovery of a previously hidden companion object”
- SupportedA black hole or neutron star companion is linked to AT 2016blu’s 113-day outbursts.NASA: “This partner is either a stellar-mass black hole, or the dense collapsed core of a massive star called a neutron star.”
- SupportedThe companion pulls material from the massive star every 113 days, triggering powerful outbursts that mimic supernovae.NASA: “the discovery of a previously hidden companion object that keeps pulling material away from the massive star every 113 days as they orbit one another.”
- SupportedThe discovery is the first X-ray detection of this kind of erupting star system.NASA: “This is the first X-ray detection of one of this class of erupting stars.”
- SupportedThe discovery offers an explanation for the 15-year puzzle surrounding AT 2016blu.NASA: ““For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis,” said Mojgan Aghakhanloo”
- SupportedMojgan Aghakhanloo said astronomers had watched the star brighten and dim regularly for 15 years and could not figure out what caused it until they got Chandra data at the right time.NASA: ““For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis,” said Mojgan Aghakhanloo of the University of Virginia in Charlottesville, who led this study. “Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen.””
- SupportedNúria Torres-Albà said the star kept acting like a supernova but survived outbursts about three times a year.NASA: ““This star’s behavior was confusing at first because it kept acting like a supernova, but clearly it’s not because it keeps surviving these outbursts about three times a year,” said co-author Núria Torres-Albà, also of the University of Virginia.”
- SupportedNathan Smith said the team suspected the repeating optical flashes might be powered by periodic blasts of high-energy X-rays.NASA: ““We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays,” said co-author Nathan Smith of the University of Arizona in Tucson.”
- SupportedAstronomers recorded at least 27 optical outbursts between 2012 and 2026, roughly every 113 days.NASA: “Between 2012 and 2026, astronomers detected at least 27 bursts of optical light, or light in wavelengths that are visible to our eyes, from AT 2016blu roughly every 113 days.”
- SupportedChandra data from March 2026 showed AT 2016blu was at least 100 times brighter in X-rays than before its outbursts began.NASA: “Chandra data from March 2026 revealed that AT 2016blu was at least 100 times brighter in X-rays than in Chandra data from 2001 to 2002, which is before the outbursts began.”
- Partly supportedThe X-ray brightness indicates that material from the star is being pulled onto a compact companion during each close orbital pass.NASA: “The best explanation for this is that material from the star is falling onto a neutron star or black hole during the closest approach in their oval-shaped orbit around one another.”
- SupportedThe X-ray brightness makes AT 2016blu a candidate ultra-luminous X-ray source, a class that is not well understood.NASA: “AT 2016blu is bright enough in X-rays to be classified as a candidate of ultra-luminous X-ray source. These are rare, bright X-ray sources found with telescopes like Chandra that astronomers have yet to fully understand.”
